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Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4-8 October2004ABN 70 592 297 967Figure 4 - Equilibrium potential-pH diagram for the Pb-H2O-CO2-SO4-Cl- system.Concentrations of species are similar to seawater. (Turgoose, 1985). Used with permissionof the United Kingdom Institute for Conservation.Table 1: Initial measured potentials across toilets 124.00009 and 124.00010ReferenceelectrodeToilet 124.00010 Toilet 124.00009Minimummeasuredpotential (mV)Maximummeasuredpotential (mV)Drivingemf (mV)Minimummeasuredpotential (mV)Maximummeasuredpotential (mV)SCE -285 -225 60 -340 -320 20Cu/CuSO4 -362 -302 60 -417 -397 20Driving emf(mV)4.1 Impressed current cathodic protectionThe aim of cathodic protection is to provide excess electrons to the surface of themetal to be protected. This limits the oxidation of the metal (anodic reaction), insteadfavouring reduction (cathodic reaction). All corrosion reactions in water involve anodic andcathodic reactions such as (1) and (2). The application of a negative potential with excesselectrons decreases the corrosion rate (Jones 1996).anodic reaction Pb (s) → Pb 2+ + 2e - (1)cathodic reaction O 2 + 2H 2 0 + 4e - → 4OH - (2)Two criteria for cathodic protection of lead are outlined in Australian Standard AS2832.1-2004© Published by the National Museum of Australia www.nma.gov.au471
Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4-8 October2004ABN 70 592 297 967Clause 2.2.2.4. “The criteria for the protection of a buried lead structure is to maintain apotential on all parts of the structure equal to, or more negative than, -650 mV with respect toa copper/copper sulphate reference electrode when the structure is in aerated conditions…”[-650 mV wrt Cu/CuSO 4 = -574 mV wrt SCE – this figure will be used henceforth.]Clause 2.2.3. “The criterion for the protection of a buried structure shall be to maintain aninstantaneous off-potential on all parts of the structure, which is at least 100 mV morenegative than the depolarized potential”However, these recommendations are for buried rather than immersed lead structures.Morgan (1987) also recommends a 100 mV negative polarisation of the potential of leadimmediately after switching off the current for a buried structure. Similarly a BritishStandard (CP 1021:1973) identifies the cathodic protection current for lead as -0.6V wrtCu/CuSO 4 without specifying if the lead is buried or immersed.The Australian Standard AS2832.3-1992 for immersed structures does not give criteria forlead specifically but states:“The accepted practice for the protection of mixed metallic structures is to maintain anegative potential at least equal to that required for the most anodic metal between allstructure surfaces and a copper/copper sulphate reference electrode.”Pourbaix (1974) notes that lead can be cathodically protected by lowering itspotential with respect to SHE to below -0.3V in acid or neutral solutions or below -0.4V to -0.8V (depending on the pH) in alkaline solutions. Despite some ambiguity across the variousstandards, the cathodic protection was set up following the criteria in clause 2.2.2.4 ofAS2832.1-2004.© Published by the National Museum of Australia www.nma.gov.au472
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Section 4: Composite artefacts (PDF 20858kb) - National Museum of ...

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